Autoclave and oven cure processing remains vital to composite structure production. One of the primary goals of the process is to fully cure a pre-impregnated thermoset polymer matrix by initiating and sustaining specific chemical reactions that relate to final resin system cure. The curing of a composite part generally requires accurate monitoring of the temperature of the part throughout the curing process. Manufacturers typically perform a large array of preproduction tests to outline the complex change in viscoelastic properties, which occurs over time and as temperature increases, simplifying it into ramp rates, hold temperatures and dwell, or soak, durations.
In the autoclave, the part is assumed to have reached full cure when certain prescribed time and temperature goals are achieved. Therefore, safety margins must be built into the time and temperature calculations to ensure full cure, and the process must be tightly controlled. Conventional autoclave control systems are hardwired to the equipment and operated by a technician, who must monitor data readouts throughout each cure cycle.
Temperature measurements and controls are typically determined using various temperature sensors placed in or around the parts and a feedback-type algorithm. The temperature sensors, for example thermocouples, may be placed inside an excess or trim region of the part or may be placed on certain locations of the associated tooling or fixtures in close proximity to the part, in order to closely track the temperature of the part throughout the process.
Various disadvantages exist for this method due to the locations of the temperature sensors. The method requires a thermal profile to be calculated, which involves significant trial and error, in order to identify a representative location on the part, and can lead to defects and failure of the part. The thermal profile is used to characterize the part temperatures and match those temperatures at locations that naturally emulate the part temperature and that do not interfere with the process or create a defect in the part. Part failure may result from failure to identify a location that adequately tracks the part temperature. Further, accurate placement of the temperature sensors in the predetermined locations and inspection of each part being fabricated is a time and labor-intensive process. Additionally, locating the temperature sensor inside the trim region of the part requires installation of a sensor for each part being cured, which is costly and may introduce defects into the parts.
Moreover, certain composite parts are exothermic during at least a portion of the autoclave or oven cure process. The exothermic properties of the composite part may affect the temperature and the thermal profile of the composite part during a cure cycle. This exothermic reaction may be substantial depending upon the thickness of the composite part.
Accordingly, those skilled in the art continue with research and development efforts in the field of thermal curing of composite parts.